Refactor answer-types to use Perseus-style scoring rather than the true/false/""/"..." mess

Summary: Doesn't change any functionality... yet... (hopefully)

Test Plan: Run http://localhost:8000/utils/test/answer-types.html and manually test answering correctly/incorrectly/unsimplified/empty for number, multiple choice, radical, multiple/set/checkbox combo, and custom in local mode.

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D4225